Appropriate expression of protein coding genes in mammalian cells requires complex nuclear choreography. Pre-mRNA processing complexes are recruited to transcription sites for capping, splicing and polyadenylation, yielding mature mRNA transcripts for nuclear export (Hirose and Manley, 2000; Lewis and Tollervey, 2000; Bentley 2005). While we understand some details about how RNA polymerase II (RNAPII) initiates transcription at promoters, or how snRNP complexes assemble into spliceosomes at splicing sites, there are gaps in our understanding about how spatiotemporal organization of gene expression within mammalian nuclei is coordinated. Nuclear speckles are domains in nuclei of higher eukaryotes enriched in pre-mRNA processing factors; dynamic exchange permits co-transcriptional pre-mRNA processing (Lamond and Spector, 2003). Our long-term goal is to understand assembly and function of nuclear speckle components and how this impacts pre-mRNA processing. Pinning down nuclear speckle functions has proven to be difficult because they contain at least 180 proteins and an undefined number of noncoding RNAs. Our lab showed that a protein called SON maintains proper organization of pre-mRNA processing factors in nuclear speckles (Sharma et al., 2010). A novel feature of SON is its unique tandem repeats that mediate proper nuclear speckle organization (Sharma et al., 2010). Two new pieces of evidence from our laboratory suggest that SON is also important for gene expression. First, SON is enriched at a facultative heterochromatin gene locus in its silent state, is removed during locus activation, and SON's repeats confer this function. Second, SON is required for appropriate alternative splicing of many gene transcripts. Our hypothesis is that SON influences gene expression via chromatin association as well as pre-mRNA splicing control for a defined subset of protein coding genes. This project aims to define SON-chromatin interactions and to design novel tools to investigate SON-dependent pre-mRNA splicing in situ.